1. Field of the Invention
The present invention is directed to light activated shape memory polymers (LASMPs), their production and use. More particularly, the current invention comprises a reaction product of a photo-reactive monomer and at least one monomer in the form of a cross-linked thermoset network in the presence of an initiator and a catalyst, structural modifier, curing agent, all, some, or none. The photo-reactive agents in these shape memory polymers allow for the shape memory effect to occur in response to different wavelengths at or near those of ultraviolet (UV), infrared (IR), or visible light, although electromagnetic waves of different wavelengths, such as radar, microwave, and X-rays, could also be used.
Shape memory is the ability of a material to remember its original shape after deformation, and return to its original shape upon activation of the shape memory effect. This phenomenon is based on a structural phase transformation. Presently the majority of shape memory polymers require activation of the shape memory effect through heat. The need for a SMP that can be activated by means other than heat is obvious to those that work in areas where heating material above ambient temperatures can be dangerous such as biomedical applications or where raising the temperature of the SMP above its transition temperatures (“Tg”) requires large amounts of energy, such as with space vehicles and aircraft. A material that can be deformed into a desired shape, and then safely returned to its original shape by exposure to light, instead of heat, would be invaluable to these applications. The present invention is also drawn to a shape memory polymer thermosetting resin having compatibility with polymers employed in high temperature, high strength and high tolerance processes in manufacturing.
Shape memory polymers (SMPs) are a unique class of polymers, which soften and harden quickly and repetitively on demand. This feature provides the ability to temporarily soften, change shape, and harden back to a solid structural state in various new highly detailed shapes and forms. Typical SMPs have a very narrow temperature span in which they transition between hard and soft states. This narrow glass transition temperature span is a key physical property that allows a SMP to maintain full structural rigidity up to a specifically designed activation temperature. Yet with as little as 5° C. to 10° C. increase above that temperature it quickly softens and allows shape change and subsequent re-hardening into new shapes.
The light activated properties of the presented polymers alters the Tg of these materials such that the Tg is below ambient temperature rather than heating the polymers to affect a shape change. Thus, instead of using heat, the application of light lowers or raises the Tg of the polymer so that the Tg of the material varies based on if the desired state of the material is hard, a higher Tg than ambient, or soft, a lower Tg than ambient.
2. Background of Prior Art
Shape memory materials are materials capable of distortion above their glass transition temperatures (Tgs), which store such distortion at temperatures below their Tg as potential mechanical energy in the material, and release this energy when heated again to above the Tg, returning to their original “memory” shape. In essence, these materials can be “fixed” to a temporary shape under specific conditions of temperature and stress and later, under thermal, electrical, or other environmental command, the associated elastic deformation can be completely or substantially relaxed to the original, stress-free, condition.
SMAs
The first materials known to have these properties were shape memory metal alloys (SMAs), including TiNi (Nitinol), CuZnAl, and FeNiAl alloys. The shape-memory capabilities of the these metallic materials capable of exhibiting shape-memory characteristics occur as the result of the metallic alloy undergoing a reversible crystalline phase transformation from one crystalline state to another crystalline state with a change in temperature and/or external stress. With a temperature change of as little as about 10° C., these alloys can exert a stress as large as 415 MPa when applied against a resistance to changing its shape from its deformed state. Such alloys have been used for such applications as intelligent materials and biomedical devices. These materials have been proposed for various uses, including vascular stents, medical guide wires, orthodontic wires, vibration dampers, pipe couplings, electrical connectors, thermostats, actuators, eyeglass frames, and brassiere underwires. With a temperature change of as little as 10° C., these alloys can exert a stress as large as 415 MPa when applied against a resistance to changing its shape from its deformed shape. However, these materials have not yet been widely used, in large part because they are very expensive. Additionally, their applications have been limited due to limited ability to withstand strains greater than approximately 8%.
SMPs
Shape memory polymers (SMPs) are being developed to replace or augment the use of SMAs, in part because the polymers are lightweight, high in shape recovery ability, easy to manipulate, and economical as compared with SMAs. SMPs are materials capable of distortion above their glass transition temperature (Tg), storing such distortion at temperatures below their Tg as potential mechanical energy, via elastic deformation, in the polymer, and release this energy when heated to temperatures above their Tg, returning to their original memory shape. When the polymer is heated to near its transition state it becomes soft and malleable and can be more easily deformed. When the temperature is decreased below its Tg, the deformed shape is fixed by the higher rigidity of the material at a lower temperature while, at the same time, the mechanical energy expended on the material during deformation will be stored. Thus, favorable properties for SMPs will closely link to the network architecture and to the sharpness of the transition separating the rigid and rubbery states.
Polymers intrinsically show shape memory effects on the basis of rubber elasticity, but with varied characteristics of temporary shape fixing, strain recovery rate, work capability during recovery, and retracted state stability. The first shape memory polymer (SMP) reported as such was cross-linked polyethylene; however, the mechanism of strain recovery for this material was immediately found to be far different from that of the shape memory alloys. Indeed, shape memory polymer is actually a super-elastic rubber. When the polymer is heated to a rubbery state, it can be deformed under resistance of ˜1 MPa modulus. When the temperature is decreased below the glass transition temperature (Tg), the deformed shape is fixed by the higher rigidity of the material at lower temperature while, at the same time, the mechanical energy expended on the material during deformation will be stored. When the temperature is raised above the Tg, the polymer will recover to its original form as driven by the restoration of network chain conformation entropy. Thus favorable properties for SMPs will be closely linked to the network architecture and to the sharpness of the transition separating the rigid and rubber states. Compared with SMAs, SMPs can withstand high strains, typically at least 200% to 400%, while the maximum strain of the SMA is typically less than 8%. As an additional advantage, due to the versatility of polymers, the properties of SMP can be tailored according to the application requirements, a factor that is very important in industry.
Several physical properties of SMPs other than the ability to memorize shape are significantly altered in response to external changes in temperature and stress. These properties include the elastic modulus, hardness, flexibility, vapor permeability, damping, index of refraction, and dielectric constant. The elastic modulus (the ratio of the stress in a body to the corresponding strain) of an SMP can change by a factor of up to 200 when heated above its melting point or glass transition temperature. Also, the hardness of the material changes dramatically when it is at or above its melting point or glass transition temperature. When the material is heated to a temperature above the melting point or glass transition temperature, the damping ability can be up to five times higher than a conventional rubber product. The material can readily recover to its original molded shape following numerous thermal cycles.
Heretofore, numerous polymers have been found to have particularly attractive shape memory effects, most notably the polyurethanes, polynorbornene, styrene-butadiene copolymers, and cross-linked polyethylene.
In the literature, polyurethane-type SMPs have generally been characterized as phase segregated linear block co-polymers having a hard segment and a soft segment. The hard segment is typically crystalline, with a defined melting point, and the soft segment is typically amorphous, with a defined glass transition temperature. In some embodiments, however, the hard segment is amorphous and has a glass transition temperature rather than a melting point. In other embodiments, the soft segment is crystalline and has a melting point rather than a glass transition temperature. The melting point or glass transition temperature of the soft segment is substantially less than the melting point or glass transition of the hard segment.
Examples of polymers used to prepare hard and soft segments of known SMPs include various polyethers, polyacrylates, polyamides, polysiloxanes, polyurethanes, polyether amides, polyurethane/ureas, polyether esters, and urethane/butadiene copolymers. See, for example, U.S. Pat. No. 5,506,300 to Ward et al.; U.S. Pat. No. 5,145,935 to Hayashi; U.S. Pat. No. 5,665,822 to Bitler et al.; and U.S. Pat. No. 6,720,420 to Langer et al.
Conventional SMPs generally are segmented polyurethanes and have hard segments that include aromatic moieties. U.S. Pat. No. 5,145,935 to Hayashi, for example, discloses a shape memory polyurethane elastomer molded article formed from a polyurethane elastomer polymerized from of a difunctional diiiosicyanate, a difunctional polyol, and a difunctional chain extender.
Recently, however, SMPs have been created using reactions of different polymers to eliminate the need for a hard and soft segment, creating instead, a single continuous piece of SMP. U.S. Pat. No. 6,759,481 to Tong, discloses such a SMP using a reaction of styrene, a vinyl compound, a multifunctional crosslinking agent and an initiator to create a styrene based SMP.
The limitations with all of these existing shape memory polymers lie in the thermal characteristics and tolerances of the material. Heat is the primary source for activating the shape memory effect in these materials. However, the Tg of the material may be too low for conditions in which the system will reside, leading to the material being incapable of activation. An example of such a situation is a hot region with an ambient temperature exceeding the transition temperature of the SMP; such a climate would not allow the polymer to efficiently make use of its rigid phase. A SMP capable of achieving a shape memory effect in such a climate would be useful. Thus there is a need for a SMP that can be activated using light or other form of electromagnetic energy.
It is an object of the present invention to provide a light activated shape memory polymer that is able to form object which can hold shape in memory in which the Tg can be tailored according to the intended application.
Another object of the invention is to provide light activated shape memory polymers that are able to form objects which can hold shape in memory in which the transition temperature and the rubbery modulus can be tailored according to the intended application and the recoverable strain can exceed several hundred percent.
It is a further object of the present invention to provide light activated shape memory polymers with physical and chemical structures that are different from those in the known shape memory polymers.
It is still a further object of the invention to provide light activated shape memory polymers that can be processed as castable formulations in the form of coatings, films and adhesives.